Neurophysiology The human brain – control & command center – Contains an estimated 100 billion nerve cells, or neurons Each neuron – May communicate with thousands of other neurons

Nervous systems consist of circuits of neurons and supporting cells All animals except sponges – Have some type of nervous system What distinguishes the nervous systems of different animal groups – Is how the neurons are organized into circuits Brain Spinal cord (dorsal nerve cord) Sensory ganglio n Salamander (vertebrate)

Structural Unit of Nervous System Neuron~ structural and functional unit Cell body~ nucelus and organelles Dendrites~ impulses from tips to neuron Axons~ impulses toward tips Myelin sheath~ supporting, insulating layer Schwann cells~PNS support cells Synaptic terminals~ neurotransmitter releaser Synapse~ neuron junction

Simple Nerve Circuit Sensory neuron: convey information to spinal cord Interneurons: information integration Motor neurons: convey signals to effector cell (muscle or gland) Reflex: simple response; sensory to motor neurons Ganglion (ganglia): cluster of nerve cell bodies in the PNS Supporting cells/glia: nonconductiong cell that provides support, insulation, and protection The three stages of information processing are illustrated in the knee-jerk reflex

The membrane potential of a cell can be measured Figure 48.9 APPLICATION Electrophysiologists use intracellular recording to measure the membrane potential of neurons and other cells. TECHNIQUE A microelectrode is made from a glass capillary tube filled with an electrically conductive salt solution. One end of the tube tapers to an extremely fine tip (diameter < 1 µm). While looking through a microscope, the experimenter uses a micropositioner to insert the tip of the microelectrode into a cell. A voltage recorder (usually an oscilloscope or a computer-based system) measures the voltage between the microelectrode tip inside the cell and a reference electrode placed in the solution outside the cell. Microelectrode Reference electrode Voltage recorder –70 mV

Neurons Have Ion Pumps & Channels K+ and Na+ maintain voltage across neuron membrane – “membrane potential” Cell is is negative inside (K+), positive outside (Na+)

The Resting Potential Is the membrane potential of a neuron that is not transmitting signals Depends on the ionic gradients that exist across the plasma membrane A neuron at rest contains many open K + channels and fewer open Na + channels in its plasma membrane. Excess K + “leaks” out of the neuron leading to more + charge outside cell and – charge inside cell. Separation of charges across the membrane = resting potential

Neuron at Work: Action Potentials Na+ & K+ channels in neuron membrane open & close when stimulated by neurotransmitters, light, pressure, sound, etc.

– + – – + – – + – – + – – + – – –– – + – + – – –– – –– – – –– – – –– –– – – – – – –– – – – – – –– – – Na + Action potential K+K+ K+K+ K+K+ Axon An action potential is generated as Na + flows inward across the membrane at one location. 1 2 The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K + flows outward. 3 The depolarization-repolarization process is repeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon. K+K+ At the site where the action potential is generated, usually the axon hillock – An electrical current depolarizes the neighboring region of the axon membrane –50 –100 Time (msec) Threshold Resting potential Membrane potential (mV) Stronger depolarizing stimulus Action potential (c) Action potential triggered by a depolarization that reaches the threshold. Depolarization  <-Repolarization

Both voltage-gated Na + channels and voltage- gated K + channels – Are involved in the production of an action potential When a stimulus depolarizes the membrane – Na + channels open, allowing Na + to diffuse into the cell As the action potential subsides – K + channels open, and K + flows out of the cell = repolarization A refractory period follows the action potential – During which a second action potential cannot be initiated Action Potential AnimationAction Potential Animation IAction Potential Animation II

The generation of an action potential – – – – – + – + – + – + – + – + – + – + – + – + – + – + – + – + – + – + Na + K+K+ K+K+ K+K+ K+K+ K+K+ 5 1 Resting state 2 Depolarization 3 Rising phase of the action potential 4 Falling phase of the action potential Undershoot Sodium channel Action potential Resting potential Time Plasma membrane Extracellular fluid Activation gates Potassium channel Inactivation gate Membrane potential (mV) –50 –100 Threshold Cytosol Figure Depolarization opens the activation gates on most Na + channels, while the K + channels’ activation gates remain closed. Na + influx makes the inside of the membrane positive with respect to the outside. The inactivation gates on most Na + channels close, blocking Na + influx. The activation gates on most K + channels open, permitting K + efflux which again makes the inside of the cell negative. A stimulus opens the activation gates on some Na + channels. Na + influx through those channels depolarizes the membrane. If the depolarization reaches the threshold, it triggers an action potential. The activation gates on the Na + and K + channels are closed, and the membrane’s resting potential is maintained. Both gates of the Na + channels are closed, but the activation gates on some K + channels are still open. As these gates close on most K + channels, and the inactivation gates open on Na + channels, the membrane returns to its resting state.

Myelinated Neurons (“white matter”) “Travel” of the action potential is self-propagating Regeneration of “new” action potentials only after refractory period Forward direction only Action potential speed: 1-Axon diameter (larger = faster; 100m/sec) 2-Nodes of Ranvier (concentration of ion channels) ; saltatory conduction; 150m/sec

Synaptic communication Presynaptic cell: transmitting cell Postsynaptic cell: receiving cell Synaptic cleft: separation gap Synaptic vesicles: neurotransmitter releasers Ca+ influx: caused by action potential; vesicles fuse with presynaptic membrane and release…. Neurotransmitter

In a chemical synapse, a presynaptic neuron – Releases chemical neurotransmitters, which are stored in the synaptic terminal Figure Postsynaptic neuron Synaptic terminal of presynaptic neurons 5 µm

Postsynaptic potentials fall into two categories – Excitatory postsynaptic potentials (EPSPs) – Inhibitory postsynaptic potentials (IPSPs)

Neurotransmitters The same neurotransmitter… Can produce different effects in different types of cells Table 48.1

The Vertebrate Brain Brainstem (mid & hindbrain) medulla oblongata, pons, & midbrain ~autonomicfunctions i.e. breathing rate, sleep cerebellum ~balance/coordination of movement Forebrain cerebrum~ complex information processing cerebral cortex~ voluntary movement & cognition corpus callosum~ connects left and right hemispheres thalamus~ main center through with sensory/motor information passes in/out of cerebrum hypothalamus~ regulates homestasis & survival;feeding, fighting, fleeing, reproduction

Sensory and Motor Mechanisms

Vertebrate Skeletal Muscle Contract/relax: antagonistic pairs w/skeleton Muscles: bundle of…. Muscle fibers: single cell w/ many nuclei consisting of…. Myofibrils: longitudinal bundles composed of…. Myofilament: Thin~ 2 strands of actin protein and a regulatory protein Thick~ myosin protein Sarcomere: repeating unit of muscle tissue, composed of…. Z lines~ sarcomere border

Sliding-filament model Theory of muscle contraction Sarcomere length reduced Z line length becomes shorter Actin and myosin slide past each other (overlap increases)

Muscle Contraction Motor neuron releases neurotransmitter (Ach) Ach binds to receptor Depolarization of muscle cell Action potential propagates along cell membrane (sarcolemma) Ca2+ released from sarcoplasmic reticulum of muslce cell which allows for… Actin binds to myosin ATP hydrolyzes actin/myosin attachment Ratcheting of myosin heads along actin during contraction Creatine phosphate~ supplier of phosphate to ADP

Muscle contraction Video

You must know… The anatomy of a neuron. The mechanism of impulse transmission in a neuron. The process that leads to release of neurotransmitter, and what happens at the synapse. The components of a reflex arc and how they work. One function for each major brain region (45.2) Examples of sensory receptors Cellular and molecular events leading to muscle contraction.